Field
The present invention relates to a semiconductor laser device capable of improving power conversion efficiency during high output power.
Background
As a light guide layer structure of a semiconductor laser device, there are structures such as an asymmetric structure in which a peak position of light confinement is displaced from a center of the light guide layer to a p-type cladding layer side and a symmetric structure in which an active layer is located at a center of the light guide layer and a peak position of light confinement is also located at the center of the light guide layer. A semiconductor laser is proposed in which an active layer is disposed at a position where light confinement of the active layer increases in the asymmetric structure than that in the symmetric structure (e.g., see Patent Literature 1: JP 2014-197598 A and Patent Literature 2: JP 2015-23180 A). This makes it possible to achieve both a reduction of a threshold current and an improvement of slope efficiency.
FIG. 28 is a diagram illustrating an optical output power-current (P-I) characteristic of a conventional asymmetric structure in comparison with a symmetric structure. The conventional asymmetric structure aims to reduce losses due to carriers in the light guide layer by displacing the position of the active layer from the center of the light guide layer to the p-type cladding layer side and improve slope efficiency. Furthermore, it is possible to reduce a threshold current by disposing the active layer at a position where light confinement of the active layer increases compared to the symmetric structure. Therefore, compared to the symmetric structure, the conventional asymmetric structure can achieve both a reduction of the threshold current and an improvement of slope efficiency.
However, when such a structure is intended to reduce an operating current during high output power and increase power conversion efficiency, the improvement of slope efficiency becomes more important than the reduction of the threshold current. This is because when the operating current is larger than the threshold current, for example, when the ratio between the two is ten times or more, the contribution of the threshold current to the power conversion efficiency is sufficiently small.
A method is also proposed which a first conductive-type low-refractive-index layer having a refractive index lower than a refractive index of a first conductive-type cladding layer is inserted between the first conductive-type cladding layer and a first conductive-type light guide layer to thereby expand a light intensity distribution in a crystal growing direction and narrow a far-field pattern (FFP) (e.g., see Patent Literature 3: JP 11-233882 A). However, it is a case where the light guide layer is thin and only a fundamental (0th-order) mode is permissible in the crystal growing direction that the light intensity distribution is expanded and the far-field pattern (FFP) is narrowed. When the light guide layer is so thick that a high-order mode equal to or higher than the first-order mode is permissible, the light intensity is conversely narrowed and the FFP expands.
The semiconductor laser device having a conventional asymmetric structure can achieve both a reduction of the threshold current and an improvement of slope efficiency, but the improvement of power conversion efficiency during high output power is limited.